Santiaguito has been erupting a hi-Na, calc-alkalic dacite since it began to grow in 1922 (Rose, 1987b). Rose (1987b) shows a cyclicity in dome extrusion rates since 1922, in which 3-5 year high extrusion rates alternate with muc hlower rates. A seventh period of increased dome extrusion began in 1986. Microprobe analyses of melt inclusion and interstitial glass in a 1902 Santa María dacite pumice (compositionally similar to Santiaguito lavas) indicate pre- and post-eruptive S contents which average 200 and 110 ppm S, respectively (Palais and Sigurdsson, 1989).
Santiaguito shows a general pattern of greater SO2 emission rates during eruptions than between eruptions (fuming). During vertical ash eruptions, which happen between 5 and 25% of the time each day, SO2 emission rates may rise to several thousand Mg/d. Between these eruptions, SO2 emission rates vary between 20 and 420 Mg/d. Considering relatively short-lived eruptive events and lengthy fuming periods, the daily Santiaguito SO2 emission rate is approximately 50 to 100 Mg/day. (Andres et al., 1993)